Taking a rationalist approach to institutions as equilibria, I develop a critical perspective on whether and when intergovernmental organizations (IGOs) promote peaceful change. I challenge the standard view that cooperation through IGOs is necessarily “peaceful” by tightening the definition of peaceful change to include not only being nonviolent and voluntary but also being noncoercive. Whether voluntary cooperation is peaceful now depends not only on the means used and end point of change but also on its starting point. Whenever prevailing institutions overly favor (previously) powerful states, seemingly cooperative change within IGOs entails implicit elements of coercion. This is especially true of formal IGOs (FIGOs) whose rules and agency are tightly tied to the interests of the powerful. By contrast, the greater flexibility of informal IGOs (IIGOs) enables them to promote change that is more inclusive of the interests of all concerned. Their greater operational capacity may give FIGOs a comparative advantage for adapting international order – and thus for peaceful change when the international order is just. But IIGOs are more effective for promoting peaceful change when larger transformational change of the international order is needed.